The present invention relates generally to fault tolerant electric power systems and, more particularly, to a fault tolerant electric power system for an active magnetic bearing having at least three axes of control and being capable of maintaining rotor shaft position in the bearing in the presence of a faulted power bus and/or control axis.
Active magnetic bearings are attracting considerable attention as a means of improving efficiency of rotating machinery, reducing or eliminating complexity of existing bearing lubrication systems, achieving high rotational speeds and gaining active control of vibrations in complex rotating masses. Recent advances in both power electronics and control microprocessors have made such active magnetic bearing systems feasible. However, for active magnetic bearings to be successfully applied to high reliability applications such as aircraft engine rotor support and vibration control, bearing and control structures which provide reliability and fault tolerance are required.
In most systems requiring high reliability, it is conventional to use brute force redundancy schemes. Such schemes have several disadvantages. For example, each multi-phase electric machine in such systems is generally tightly coupled electromagnetically from phase to phase, preventing it from functioning as a fault tolerant component by itself via phase redundancy. Since each individual machine lacks fault tolerance, multiple independent machines must be combined to provide the required system redundancy, resulting in added weight, volume and potential failure points. Another disadvantage is that each multi-phase power electronic drive/controller is tightly coupled electrically from phase to phase, preventing it from functioning as a fault tolerant component by itself via phase redundancy. Since each individual drive/controller lacks fault tolerance, multiple independent drives/controllers must be combined to provide the required system redundancy resulting again in added weight, volume and potential failure points. Still another disadvantage is that the multiple independent machines and drives/controllers are interconnected electrically by a common bus and/or mechanically by a common shaft so that the failure of one provides paths for the fault to propagate to other parts of the system. Such fault propagation negates the power system redundancy, resulting in loss of power to the load. Thus, overall reliability of the electric power system depends critically on the failure rates of the electric machines.
While alternating current (AC) machines are not inherently fault tolerant, a switched reluctance (SR) machine can be used as both a motor and a generator and does have inherent fault tolerant capabilities. A basic three-phase SR machine uses iron laminations with salient stator poles and rotor poles. The number of stator poles is greater than the number of rotor poles. For example, one form of machine may have twelve salient stator poles and eight rotor poles. The stator windings fit around the stator poles and are arranged in pairs or phases so that the rotor poles are attracted towards alignment with the stator poles when the stator poles are excited. This operation allows for either positive or negative torque reduction, depending on whether the rotor is approaching or leaving alignment with a stator pole. The inherent simplicity of the basic SR machine makes it desirable for aerospace applications. The rotor comprises a stack of iron laminations with no spinning magnets or windings, making the machine usable at high speed. Further, since the SR machine has no magnets, it can be designed to operate at high temperatures limited by the temperature ranges of available wire insulation systems. Another feature which makes an SR machine attractive for aerospace applications is its inherent fault tolerance. The SR stator phases are to a high degree electromagnetically independent of each other. This fact, combined with the absence of spinning rotor magnets, allows the SR machine to sustain a serious fault in any stator winding without disrupting torque production in the other unfaulted phases. These fault tolerant properties extend to the drive power electronics in that the SR machine stator windings are excited as separate phase legs rather than being interconnected in wye or delta configurations as in conventional AC drives.
As a result of the phase independence in an SR machine, the available average torque output from an N-phase SR machine, following loss of a phase, is simply (N-1)/N of its prefault value (excluding compensating actions). Thus, increasing the number of phases reduces the post-fault output degradation. For example, post-fault torque production is 67 percent for a single phase failure in a three-phase machine and 75 percent for a single phase failure in a four-phase machine. In addition, the average torque lost in the faulted phase can be replaced by designing the SR machine and its control electronics to permit elevated excitation of the remaining unfaulted phases. Therefore, the basic SR machine, due to the inherent electrical and magnetic phase independence, can continue to function in the presence of power electronic and electrical machine faults. An SR motor drive, once started, can continue to operate with as little as one phase powered.